Mature cystic teratoma is the most common ovarian neoplasm and affects mostly young patients. It is a benign germ cell tumour consisting of at least two of the three embryogenic germ cell layers, and usually contains ectodermal (skin, brain), mesodermal (fat, bone) and/or endodermal (thyroid tissue, gastrointestinal and bronchial epithelium) mature tissue. Simultaneous presence of these components leads to a complex and heterogeneous appearance. However, the key to a correct diagnosis is detection of fat-tissue within the mass. MRI usually demonstrates tissue with high signal intensity both on T1-weighted and T2-weighted images and signal loss on fat suppression sequences. A T1-weighted fat-saturated sequence is also able to make differential diagnosis between fat-tissue and haemorrhagic lesions, namely endometrial cysts. A fat-fluid interface may also be found, and is typical of mature cystic teratoma. MRI can also demonstrate complex masses with multiple cystic and solid areas. Cystic spaces may show variable signal intensity demonstrating both high and low signal intensity on T1-weighted and T2-weighted images. In particular, some cystic spaces demonstrate low signal intensity on both T1-weighted and T2-weighted images because of the colloid of the struma.